This invention relates to separation of the components of immiscible liquid mixtures, such as oil and water, wherein one of the liquids (the oil) is finely dispersed in the other liquid. More particularly, the present invention is directed to improved processes and apparatus making novel use of the combined effects of specific gravity differences of the immiscible liquids, substantially static conditions maintained with the pores of a mixture holding and transport matrix, and the "catalytic" effect of the pore surfaces selectively attractive to the liquid to be coalesced. The resultant mixture thus transported by the matrix to a point of discharge while coalescence is occurring is thereby conditioned for separation of the coalesced first liquid from the second liquid by other physical processes and apparatus known in the art.
Environmental protection efforts in recent years have yielded various proposals for separating oil from water. Most techniques operated on the assumption that the oil is already stratified, i.e., either floating as a layer on the water or existing in sizable globules readily settled out of suspension or readily contacted by filter element surfaces. Most of the floating oil existing in that condition can be recovered by any of different techniques including skimming, decanting, contact filtration or contact absorption.
In many situations, however, oil pollution of water exists wherein the oil is dispersed in droplets of such minute size that bulk or stratified oil separation techniques are ineffective. Typical of these are waste or by-product water discharged from manufacturing facilities such as oil refineries, oil/water mixtures pumped centrifugally against substantial heads and resulting in oil dispersion, engine cooling water discharge, industrial or commercial establishment run-off water where oil is present, ships bilge water and ships ballast water. Under the increasingly rigid anti-pollution policies of industry and government, even the slightest traces of oil are objectionable; for example, as little as 10 parts per million of oil in water. This is true not only with oil spills on open bodies of water but often in fixed installations wherein large volumes of water containing small traces of oil must be purified economically. Chemical treatment to precipitate the oil or to convert it into an acceptable material is usually not economical. Flow-through filters act too slowly and are not effective to coalesce minute oil particles.
For example, in seemingly quiescent settling ponds or tanks, naturally occurring thermal convection or other disturbances, however, small, tend to keep the particles or droplets continuously in motion. Those that settle to the surface of the water are extremely slow to coalesce. The relatively very high surface tension acting upon such small oil droplets tends to keep the droplets apart and thwarts coalescence.
Flow-through filter type coalescers are also of limited effectiveness. The light, minute oil droplets are extremely "motile" i.e., motion-reactive to the slightest pressure gradients in the surrounding water and thereby avoid contact with surfaces. Viscous boundary layer forces attending flow, however slow, along the filter matrix surfaces tend to keep the oil droplets away from the surfaces owing to velocity gradients and attendant pressure gradients. Further, matrix surface irregularities causing adjacent flow path curvature force the lighter oil droplets away from the intended coalescing surfaces through centrifugal force effects. Since these are both velocity-dependent effects, attempts to increase mixture processing rates in conventional flow-through coalescing filters are counter-productive. Capacity increase effected through increased filter element cross-sectional dimensioning, or by adding banks of filters, adds undesirably to the bulk and cost of the apparatus as a whole. Flow-through filters with matrix channels smaller than oil droplets diameter can effectively coalesce oil, but tend to plug up so as to require frequent cleaning or replacement of filter elements.
A further object herein is to overcome the problems described, and to devise a coalescer process utilizing an apparatus which is essentially self-reconditioning, continuously and reliably operable unattended substantially over long time periods, and relatively non-critical in terms of operating capability as affected by particle size or mixture proportions.
A further object hereof is to achieve the stated objectives with process concepts that offer versatility in selecting the overall physical size and shape of equipment to suit varying installation and operation requirements. Thus scaling up or scaling down the size to vary the volumetric capacity proportionately may be done without adversely affecting operating efficiency or effectiveness.